Thunder is the loud noise that comes right after lightning. It also can refer to a loud, booming sound, or mean to move heavily but quickly.
Thunder is made up of the rumbles and cracks accompanying lightning during a thunderstorm. You can also use this word for other loud noises, like the thunder of a loud rock band or your uncle's booming voice. A powerful singer can be said to thunder, singing in a way that's as noisy and impressive as a thunderstorm. Thunder can also be used for big, fast movements, like a tank thundering across a field or a truck thundering down the highway.